Populations of surface-associated bacteria are commonly referred to as biofilms. In most natural settings bacteria are found predominantly in biofilms, yet for many years studies of bacterial physiology focused primarily on the planktonic state. The widespread recognition that biofilms impact myriad environments, from water pipes to indwelling devices in hospital patients, has led to an increase interest in investigating the molecular mechanisms underlying the formation and maintenance of these communities. From the diversity of biofilm formation strategies thus far described emerges much knowledge that allows us to formulate a general hypothesis for this phenomenon: "Biofilm formation is a developmental process in which bacteria undergo a regulated lifestyle switch from a nomadic unicellular state to a sedentary multicellular state where subsequent growth results in structured communities and cellular differentiation." The experiments proposed here represent our efforts to critically test aspects of this central hypothesis using the sporulating bacterium, Bacillus subtilis. For motile bacteria, the lifestyle switch from a nomadic to a sedentary existence usually entails the shutdown of motility with the concomitant synthesis of the extracellular matrix. While it is generally agreed that matrix production and motility are mutually exclusive, the molecular mechanisms that underlie the lifestyle switch remain virtually unknown for most bacterial species. Through a combination of genetic and biochemical approaches, we have begun to identify the molecular regulatory circuitry that governs the transition from motile cells to matrix-enclosed sedentary communities of B. subtilis. We will continue to apply genetic and biochemical approaches to address questions regarding: (1) The molecular mechanism via which a master regulator controls the lifestyle switch, (2) The assembly of the extracellular matrix, and (3) The spatio-temporal organization and cellular differentiation processes that occur within the biofilm.